Advances in polymerization and catalysts have produced new polymers having improved physical and mechanical properties useful in a wide variety of products and applications. With the development of new catalysts, the choice of polymerization, such as solution, slurry, high pressure, or gas phase, for producing a particular polymer has been greatly expanded. Advances in polymerization technology have also provided more efficient, highly productive and economically enhanced processes.
Multimodal polymers are known to be useful for making a variety of commercial products such as films, pipe, and blow molding products. In general, a multimodal polymer has at least two (e.g., two, three or more) molecular weight fractions, and a molecular weight distribution that determines a different identifiable molecular weight for each molecular weight fraction. For example, a multimodal polymer may have at least one low molecular weight fraction and at least one high molecular weight fraction, and a molecular weight distribution that determines an identifiable (relatively high) molecular weight for the high molecular weight fraction and another identifiable (relatively low) molecular weight for the low molecular weight fraction. A composition comprising only two molecular weight fractions is referred to as “bimodal.”
A number of different techniques have been developed to produce multimodal polymers. One such technique uses a suitable polymerization catalyst to broaden the molecular weight distribution in the resultant polymer. Multi-stage polymerization processes have also been used in which different hydrogen concentrations may be used in the different stages to broaden the molecular weight distribution of the resultant polymer. Examples of multi-stage polymerization processes include slurry-phase-slurry-phase processes, gas-phase-gas-phase processes, and slurry-phase-slurry-phase processes. One particular process for producing a bimodal polymer utilizes a loop reactor and a gas-phase reactor arranged in series. Despite their use in commercial products, multimodal polymers still desire improvements in their production processes as current processes may lack adequate molecular weight control. Improved control of molecular weight control may beneficial for producing polyolefins suitable for a number of different applications, including blown and cast film applications.